The present invention relates to an active matrix type liquid crystal display device (AM-LCD) driven by thin film transistors (TFT) and a method of manufacture thereof.
In an effort to provide image display devices that have a reduced thickness and weight and are able to attain high definition, the market has moved toward the provision of thin film transistor-driven liquid crystal display devices (TFT-LCD), instead of the existing cathode ray tubes. The conventional TFT-LCD comprises scanning signal lines, image signal lines, thin film transistors formed in the vicinity of intersections between the scanning signal lines and the image signal lines, pixel electrodes connected with the thin film transistors, a gate insulative film and a protection film formed on a glass substrate; and, a counter substrate is provided with a liquid crystal layer disposed between the glass substrate and the counter substrate. In recent years, along with an increased size and an improved definition of the screen of a TFT-LCD, specifications required for a reduction in the resistance of scanning signal lines and image signal lines and an increase in the production yields have become more and more severe. Further, there has also been a demand to reduce the production cost by simplifying the production steps.
For the image signal lines of the bottom gate type amorphous silicon TFT-LCD, those comprising a single layer of a metal film, such as Ti, Ta, Cr, Mo and CrMo, or those made of laminated metal films, such as Mo/Cr, Al/Ti, CrMo/Cr, Mo/Al/Mo, Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr, have been adopted. In the foregoing, the slash (/) represents the constitution of laminated films in which an upper layer is indicated on the left and a lower layer is indicated on the right of the slash. Such wiring constitutions are properly selected depending on the wiring resistance specifications required for liquid crystal driving, the production performance of a sputtering step, the performance of an etching apparatus, and so on.
Among the various possibilities, the constitution capable of obtaining the lowest wiring resistance can include films employing Al, Al/Ti, Mo/Al/Mo, Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr, and a similar constitution capable of obtaining the second lowest wiring resistance can include employing pure Mo, Mo and Mo/Cr. Incidentally, Ti or Mo can be laminated as an upper or a lower layer on Al, such as in Ti/Al/Ti or Mo/Al/Mo, in order to improve the contact with the silicon constituting a thin film transistor and a pixel electrode comprising indium oxide or the like as the main ingredient. Further, those simple constitutions providing the lowest load on the sputtering step can include single-layered constitutions of Ti, Ta, Cr, Mo or CrMo, and similar constitutions for providing the second lowest load can include a two-layered constitution of Mo/Cr, Al/Ti. Further, the constitutions capable of selective wet etching for a gate insulative film without the use of a hydrofluoric system etching solution can include Cr, Mo, CrMo, Al/Cr, Mo/Al/Mo, Cr/Al/Cr and MoCr/Al/MoCr. Further, those constitutions capable of forming through holes in the protection insulative film substantially covering a drain electrode and a source electrode without eliminating the wiring film can include Cr, CrMo, Mo/Cr, CrMo/Cr, Cr/Al/Cr and MoCr/Al/MoCr.
The constitutions of the image signal lines described above have respective features, as described previously, but constitutions capable of simultaneously satisfying requirements for reduced wiring resistance, a reduced load on the sputtering step, selective wet etching for the gate insulative film, and formation of the through hole in the protection insulative film by dry etching, have not yet been known.
For example, Ti has a high resistivity, and since buffered hydrofluoric acid is used for wet etching for Ti, selective wet etching with respect to the gate insulative film is difficult. Further, Ta also has a high resistivity, and since buffered hydrofluoric acid is used for wet etching for Ta, selective wet etching with respect to the gate insulative film is also difficult. Further, since it is etched with an SF6, gas, formation of the through holes in the protection insulative film by dry etching is difficult. Further, Cr, CrMo, and CrMo/Cr each have a high resistivity. Also, since Mo is etched by SF6, gas, formation of the through hole in the protection insulative film by dry etching is difficult. Since a Mo layer is rapidly dissolved abnormally upon wet etching in Mo/Cr, patterning is difficult. Since it is necessary for Al/Ti that the through hole has to be formed by dry etching in the protection insulative film and Al has to be etched, the step is complicated. Further, for Mo/Al/Mo, since Mo as a cap layer is etched by SF6 gas, and formation of the through hole in the protection insulative film by dry etching is difficult and also requires three-layered deposition, the sputtering step is subjected to a large load. Similarly, since Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr each require the deposition of three layers, they impose a great load on the sputtering step.
A first object of this invention is to attain a constitution of image signal lines capable of simultaneously satisfying the requirements of reduced wiring resistance, a reduced load on a sputtering step, selective wet etching relative to a gate insulative film, formation of a through hole in a protection insulative film by dry etching, and favorable contact with silicon of a thin film transistor and a transparent conductive film constituting a pixel electrode, and to provide a liquid crystal display device using the same. An target value for the wiring resistance is assumed to be 170 nxcexa9/m or lower of an average conductivity defined as a product of sheet resistance and overall film thickness. This average conductivity is at a level lower than 180 nxcexa9/m that can be attained with a single Cr layer film or single CrMo layer film.
A target for reduction in the load imposed on the sputtering step involves setting the number of laminated layers to two or less. The selective wet etching with respect to the gate insulative film is such that etching can be conducted with a chemical solution excluding those capable of chemically attacking the gate insulative film, such as a buffered hydrofluoric acid or an alkali solution giving damage to a resist. Further, from the viewpoint of the simplicity of the steps, it is a necessary condition that upper and lower layers can be etched simultaneously on a wiring constituted of laminated layers. Further, it is a necessary condition that the cross section of the wirings can be fabricated into a tapered shape. For the formation of the through hole in the protection insulative film by dry etching, it is necessary that the contact layer of wirings has a durability to dry etching using an SF6 gas for establishing contact between the wirings and the pixel electrode. As a matter of fact, it is difficult to use an alloy comprising Al as a main ingredient for the contact layer.
A second object of this invention is, in addition to attaining the foregoing first object, to provide a good matching property with a process for forming scanning signal lines. Since a particularly lowered resistance is required for the scanning signal lines, it is desirable that a film constituting them is a laminated film containing an alloy comprising aluminum as the main ingredient. When the etching solution for fabricating the same and that for fabricating the image signal lines can be used in common, this is preferred in view of the consequent reduction in the manufacturing cost.
According to a first aspect of the present invention, a liquid crystal display device comprises: a pair of substrates; a liquid crystal layer disposed between the pair of substrates; a plurality of scanning signal lines formed on one of the pair of substrates; a plurality of image signal lines crossing the scanning signal lines in a matrix arrangement; thin film transistors formed in the vicinity of intersections of the scanning signal lines and the image signal lines; pixel electrodes connected with the thin film transistors; a gate insulative film substantially covering the scanning signal lines; and a protection insulative film substantially covering the image signal lines and the thin film transistors. At least one of the signal lines of the scanning signal lines and the image signal lines is made of a two-layered film of a first conductive film as a lower layer and a second conductive film as an upper layer, and the first conductive film is made of an alloy comprising molybdenum as a main ingredient and contains tungsten, while the second conductive film is made of an alloy comprising molybdenum as a main ingredient and contains zirconium.
Preferably, the first conductive film may be made of an alloy comprising molybdenum as a main ingredient and containing tungsten, and the second conductive film may be made of an alloy comprising molybdenum as a main ingredient and containing 4% by weight or more of zirconium.
Preferably, the average conductivity defined as a product of the sheet resistance and the entire film thickness of the two-layered film may be 170 nxcexa9/m or less.
Preferably, a cross section at a fabrication end of the first conductive film may have a forwardly tapered shape.
According to a second aspect of the invention, a liquid crystal display device comprises: a pair of substrates; a liquid crystal layer disposed between the pair of substrates; a plurality of scanning signal lines formed on one of the pair of substrates; a plurality of image signal lines crossing the scanning signal lines in a matrix arrangement; thin film transistors formed in the vicinity of intersections between the scanning signal lines and the image signal lines; pixel electrodes connected with the thin film transistors; a gate insulative film substantially covering the scanning signal lines; and a protection insulative film substantially covering the image signal lines and the thin film transistors. The image signal lines and a source and drain electrodes of the thin film transistor are made of a two-layered film of a first conductive film disposed as a lower layer and a second conductive film disposed as an upper layer, the first conductive film being directly connected with silicon constituting the thin film transistor, while the second conductive film is directly connected with the pixel electrode by way of a through hole disposed in the protection insulative film. The first conductive film is made of an alloy comprising molybdenum as a main ingredient and containing tungsten, and the second conductive film is made of an alloy containing zirconium.
Preferably, the first conductive film may be made of an alloy comprising molybdenum as a main ingredient and containing tungsten, and the second conductive film may be made of an alloy comprising molybdenum as a main ingredient and containing 4% by weight or more of zirconium.
Preferably, the average conductivity defined as a product of the sheet resistance and the overall film thickness of the two layered film may be 170 nxcexa9/m or less.
Preferably, a cross section at a fabrication end of the first conductive film may have a forwardly tapered shape. Preferably, the scanning signal lines may be made of a laminated film of an alloy comprising aluminum as a main ingredient and an alloy comprising molybdenum as a main ingredient.
Preferably, the pixel electrode may be made of a mixed oxide of indium oxide, tin oxide and zinc oxide.
According to a third aspect of the invention, a liquid crystal display device comprises: a pair of substrates; a liquid crystal layer disposed between the pair of substrates; a plurality of scanning signal lines formed on one of the pair of substrates; a plurality of image signal lines crossing the scanning signal lines in a matrix arrangement; thin film transistors formed in the vicinity of the intersections of the scanning signal lines and the image signal lines; a gate insulative film substantially covering the scanning signal lines; a protection insulative film substantially covering the image signal lines and the thin film transistors; and at least a pair of pixel electrodes and counter electrodes formed on one of the pair of substrates within a plurality of pixels formed in regions surrounded with the plurality of scanning signal lines and the plurality of image signal lines. An image signal is supplied to the pixel electrode by way of the thin film transistor that is driven based on the supply of a scanning signal from the scanning signal line, and a reference voltage is supplied to the counter electrode by way of the counter voltage signal line formed over the plurality of pixels. The pixel electrode is made of a two-layered film of a first conductive film disposed as a lower layer and a second conductive film disposed as an upper layer, and the first conductive film is made of an alloy comprising molybdenum as a main ingredient and containing tungsten, while the second conductive film is made of an alloy containing zirconium.
Preferably, the first conductive film may be made of an alloy comprising molybdenum as a main ingredient and containing tungsten, and the second conductive film is made of an alloy comprising molybdenum as a main ingredient and containing 4% by weight or more of zirconium.
Preferably, the average conductivity defined as a product of the sheet resistance and the overall film thickness of the two layered film may be 170 nxcexa9/m or less.
Preferably, a cross section at a fabrication end of the first conductive film may have a forwardly tapered shape.
According to a fourth aspect of the invention, a liquid crystal display device comprises: a pair of substrates; a liquid crystal layer disposed between the pair of substrates; a plurality of scanning signal lines formed on one of the pair of substrates; a plurality of image signal lines crossing the scanning signal lines in a matrix arrangement; thin film transistors formed in the vicinity of the intersections between the scanning signal lines and the image signal lines; pixel electrodes connected with the thin film transistors; a gate insulative film substantially covering the polycrystal silicon of the thin film transistors; an interlayer insulative film for substantially covering the scanning signal lines and insulating the scanning signal lines from the image signal lines; and a protection insulative film substantially covering the image signal lines. The gate electrodes of the thin film transistors may be made of a two-layered film consisting of a first conductive layer disposed as a lower layer and a second conductive layer disposed as an upper layer. The first conductive film is made of an alloy comprising molybdenum as a main ingredient and containing tungsten, and the second conductive film is made of an alloy containing zirconium.
The object of this invention can be attained by the constitutions described above. The concept of the invention will be described below.
Metal elements with relatively low resistivity include, Ag, Cu, Al, Mo and W, when poisonous elements and those elements which are considered to be difficult for film deposition by sputtering, such as noble metals, alkali metals, alkaline earth metals and ferromagnetic materials, are excluded from the metal elements in the periodical table. Among them, Ag, Cu and Al each have poor contact with the silicon used in a thin film transistor or a transparent conductive film that constitutes a pixel electrode and they require a so-called barrier metal or cap metal. That is, for connecting Ag, Cu or Al with silicon and a transparent conductive film, it is necessary to laminate Mo or the like on an upper layer and a lower layer of Ag, Cu or Al. In this case, the metal film has a three-layered constitution, which remarkably increases the load imposed on a sputter film deposition step.
Further, W requires a buffered hydrofluoric acid or alkali solution for wet etching fabrication. However, the buffered hydrofluoric acid damages the glass substrate or the SiN of the gate insulative film, while the alkali solution damages the resist, so that it is difficult to use them for the wiring forming step.
As a result of the considerations described above, Ag, Cu, Al and W have been judged not suitable to the object of the invention, while the remaining Mo and alloys thereof with a high melting metal (Ti, Cr, Zr, Nb, Hf, Ta and w) require no barrier metal or cap metal and can be wet etched by using a mixed acid of phosphoric acid-nitric acid. However, pure Mo has no durability to SF6 dry etching upon fabrication of through holes in the SiN film. Further, it is difficult to fabricate the wiring cross section into a tapered shape by wet etching. In view of the above, laminating and alloying of the wiring film are considered with the following approach.
At first, for forming the wiring cross section into a tapered shape, the upper layer, in the two-layered film, is provided with a relatively high wet etching rate and is formed as a relatively thin film, while the lower layer is provided with a relatively low wet etching rate and is formed as a relatively thick film. Further, the upper layer is provided with durability to dry etching, while the lower layer is adapted to serve as a conductive layer. That is, the upper layer is required for a high wet etching rate and high dry etching durability, while the lower layer is required for a low wet etching rate and low resistivity. The present inventors have made a study of the property of alloys formed by adding a second element to Mo and have found Mo alloys capable of satisfying the foregoing required characteristics.
FIG. 15 shows a wet etching rate and SF6 dry etching rate of Mo alloys with addition of Ta, Cr, Ti, Hf and Zr, respectively, to Mo. For each of the additive elements, the wet etching rate and the dry etching rate are lowered as the amount of additive increases. Although not illustrated, when W or Nb is added to Mo, a similar trace as in the case of adding Ta is obtained. Among the additive elements, Zr is an additive element having the greatest ratio with respect to the amount of reduction of the dry etching rate relative to the amount of reduction of the wet etching rate, and Hf is next to Zr. When the material cost is considered for Zr and Hf, it is apparent that Hf is outstandingly expensive. Accordingly, the optimum element as the upper layer for the two-layered film is an alloy formed by adding Zr to Mo. In order to provide the image signal line with the required dry etching durability, it is desirable that Zr is added by 4% by weight or more.
FIG. 16 shows the wet etching rate and the resistivity of the Mo alloys with addition of W, Ta, Nb, Hf and Zr, respectively, to Mo. In each of the additive elements, the wet etching rate lowers and the resistivity increases as the amount of additive increases. Among the additive elements, W is an additive element showing a minimum ratio of increasing resistivity relative to the amount of reduction of the wet etching rate, and Ta is next to W. Although not illustrated, Zr and Hf are additive elements showing an extremely large ratio of increasing resistivity relative to the amount of reduction of the wet etching rate. In view of the foregoing, the optimal lower layer for the two-layered film is made of an alloy in which W is added to Mo. The amount of W added in the MoW alloy as the lower layer depends on the amount of Zr added in the MoZr alloy for the upper layer. That is, in order to provide a wiring cross section having a favorable tapered shape, W has to be added within a range such that the wet etching rate for the lower layer is less than that for the upper layer and at an addition amount as low as possible so as to attenuate a rise in the resistivity.
FIG. 17 illustrates a cross sectional shape of a two-layered film comprising the MoZr alloy for the upper layer and the MoW alloy for the lower layer, as described above, which is subjected to wet etching. It is wet etched into a tapered cross sectional shape in which the MoZr layer disposed as the upper layer is retracted. The etching solution is a mixed acid containing phosphoric acid -nitric acid used as an aluminum etching solution, and when a film containing an aluminum alloy is used for the scanning signal lines, the etching solution can be used in common.
Further, when a film containing an aluminum alloy is used for the scanning signal line, the yield in view of the disconnection of the scanning signal line is worsened when polycrystal indium tin oxide is adopted for the pixel electrode. This is because dissolution disconnection is caused by intrusion of an intense halogenic acid, such as hydrobromic acid, as an etching solution for the polycrystal indium tin oxide as far as scanning signal lines. Dissolution disconnection is essentially eliminated by adopting an amorphous transparent conductive film, such as one made of a mixed oxide of indium oxide, tin oxide and zinc oxide, as the pixel electrode and changing the etching solution with a mild etchant, such as oxalic acid. Contact characteristics between a mixed oxide of indium oxide, tin oxide and zinc oxide and the MoZr alloy described above are favorable, and the transparent conductive film can be adopted as the pixel electrode.
In the case of the scanning signal lines, since there is no requirement for considering contact with silicon, an aluminum alloy of lower resistance can be adopted as the lower layer for the wiring film. However, when the polycrystal indium tin oxide is used as the pixel electrode, as described above, the use of an aluminum alloy is not desirable in view of the disconnection yield. The advantage of adopting the polycrystal indium tin oxide is that the connection resistance is low and stable at the wiring terminal formed simultaneously with the pixel electrode. In this case, it is possible to apply the two-layered film constitution, in which the upper layer is made of the MoZr alloy and the lower layer is made of the MoW alloy as described above, to the scanning signal line. As described above, in the case of the two-layered film constitution, since the tapered shape at the fabrication end is satisfactory, the coverage and the image signal line overriding characteristics of the gate insulative film are improved.